7th ICCK (MIT, Cambridge) July 11, 2011 1 Molecular Size Dependent Molecular Size Dependent Fall-off Rate Constants Fall-off Rate Constants for the Recombination for the Recombination Reactions of Alkyl Reactions of Alkyl Radicals with O Radicals with O 2 2 Akira Miyoshi Akira Miyoshi Department of Chemical Systems Engineering, Department of Chemical Systems Engineering, University of Tokyo University of Tokyo
Molecular Size Dependent Fall-off Rate Constants for the Recombination Reactions of Alkyl Radicals with O 2. Akira Miyoshi. Department of Chemical Systems Engineering, University of Tokyo. Introduction. — R (alkyl) + O 2. - PowerPoint PPT Presentation
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7th ICCK (MIT, Cambridge) July 11, 2011
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Molecular Size Dependent Molecular Size Dependent Fall-off Rate Constants for Fall-off Rate Constants for
the Recombination the Recombination Reactions of Alkyl Radicals Reactions of Alkyl Radicals
with Owith O22
Akira MiyoshiAkira Miyoshi
Department of Chemical Systems Engineering, University of TokyoDepartment of Chemical Systems Engineering, University of Tokyo
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IntroductionIntroduction
— — R (alkyl) + OR (alkyl) + O22
•• key reactions that lead to chain branching in low-temperature key reactions that lead to chain branching in low-temperature oxidation of hydrocarbonsoxidation of hydrocarbons
— — ChallengesChallenges
•• resolution of complicated pressure- and temperature- dependent resolution of complicated pressure- and temperature- dependent product specific rate constants including second Oproduct specific rate constants including second O22 addition addition
reactions to QOOHreactions to QOOH
— — ObjectivesObjectives
•• evaluation of universal fall-off rate expression for recombinationevaluation of universal fall-off rate expression for recombination•• master equation analysis for the dissociation/recombination master equation analysis for the dissociation/recombination
steady-statesteady-state
ComputationalComputational
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ComputationalComputational
— — Quantum Chemical CalculationsQuantum Chemical Calculations
•• B3B3LYP & CBS-QB3 calculations by Gaussian 03LYP & CBS-QB3 calculations by Gaussian 03•• CASPT2 calculations by MOLPRO 2008.1CASPT2 calculations by MOLPRO 2008.1
— — TST and VTST CalculationsTST and VTST Calculations
by by GPOP*GPOP* including: including:•• PPitzer-Gwinn approximation for hindered rotors, itzer-Gwinn approximation for hindered rotors, qqPGPG
(after analysis by (after analysis by BEx1D*BEx1D*))•• 1D tunneling correction (asymmetric Eckart), 1D tunneling correction (asymmetric Eckart), κκtuntun
•• rotational conformer distribution partition function, rotational conformer distribution partition function, qqRCDRCD
— — RRKM/ME CalculationsRRKM/ME Calculations
•• ρρ((EE) and ) and kk((EE) accounting for all TST feature () accounting for all TST feature (qqPGPG, , κκtuntun, and , and qqRCDRCD) ) by by
•• CCASPT2(7,5)/aug-cc-pVDZ // ASPT2(7,5)/aug-cc-pVDZ // B3LYP/6-311G(d,p) potential B3LYP/6-311G(d,p) potential energy well reproduced energy well reproduced experimental experimental kk(300 K) (300 K) within within ± 25%± 25%
•• B3LYP/6-311G(d,p) potential B3LYP/6-311G(d,p) potential energy systematically energy systematically underestimated underestimated kk(300 K)(300 K)
10 High-Pressure Limiting Rate Constants, High-Pressure Limiting Rate Constants, kk
same for primary R'ssame for primary R's
same for secondary R'ssame for secondary R's
•• class (class (primaryprimary, , secondarysecondary, or , or tertiarytertiary) ) determines the rate determines the rate constantconstant
— — Size-IndependentSize-Independent
— — Class-SpecificClass-Specific
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Fall-off CalculationsFall-off Calculations
Plumb & Ryan, Int. J. Chem. Kinet., 1981, 13, 1011; Slagle et al., J. Phys. Chem., 1984, 88, 3648; Wagner et al., J. Phys. Chem., 1990, 94, 1853.
— — Energy Transfer Energy Transfer ModelModel
0.7
1400
1000 Kcm
T
•• experimental data for experimental data for CC22HH55 + O + O22 in fall-off in fall-off region were well region were well reproduced by the reproduced by the exponential-down exponential-down model with:model with:
— — Near Boltzmann DistributionNear Boltzmann Distribution
•• rate constants for subsequent isomerization/dissociation reactions rate constants for subsequent isomerization/dissociation reactions of ROof RO22 can be estimated to be in near high-pressure limit can be estimated to be in near high-pressure limit
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Three "Steady-States"Three "Steady-States"
Miller and Klippenstein, Int. J. Chem. Kinet., 2001, 33, 654–668.
Clifford, Farrell, DeSain and Taatjes, J. Phys. Chem. A, 2000, 104, 11549–11560.
"prompt"
"delayed"
20 HOHO22 formation in C formation in C22HH55 + O + O22 C C22HH55OO22
Experimental data by Clifford, Farrell, DeSain and Taatjes, J. Phys. Chem. A, 2000, 104, 11549–11560.
•• kk((HOHO22) ) kk((HOHO22))at moderate at moderate TTbut in partial equilibrium ofbut in partial equilibrium ofR + OR + O22 RO RO22
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Time Dependent SolutionTime Dependent Solution
indd
kt
n Jn r
Time-dependent solution forTime-dependent solution for
with with nn00 = = 00 and and kkinin = const. = const.
Build-up timeBuild-up time kkdis,FOdis,FO–1–1
•• Nearly the same with and Nearly the same with and without concerted HOwithout concerted HO22 elimination channelelimination channel